The invention relates to straining, screening or filtering machines in general, and more particularly to improvements in machines for intercepting solid constituents in liquid streams. For example, the machine of the present invention can be utilized to intercept and remove solid debris and/or other solid constituents from flowing sewage.
It is already known to intercept solid constituents in liquid streams by placing a sloping screen into a body of flowing liquid in such a way that the lower end of the screen rests on the bottom of the liquid conveying channel and the upper end of the screen is located at a level above the body of liquid. The outer side of the screen faces counter to the direction of liquid flow therethrough, and the screen is assembled of two sections or grates each having a set of parallel grate bars extending from the lower end to the upper end of the screen. The grate bars of one grate alternate with the grate bars of the other grate. One of the grates is stationary, and the other grate is movable relative to the one grate forwardly (counter to the direction of liquid flow through the screen) to lift intercepted impurities off the grate bars of the stationary grate, thereupon upwardly to raise the lifted impurities toward or above the liquid level, and thereupon rearwardly or inwardly (in the direction of flow of the liquid stream). This results in stepwise lifting of intercepted impurities along the exposed side of the stationary grate.
The mechanism which is used to move the mobile grate longitudinally of the grate bars as well as transversely of the stationary grate (in and counter to the direction of liquid flow through the screen) in a presently known machine of the above outlined character comprises rollers which are secured to the mobile grate at each side of the screen and are compelled to move along tracks which are provided therefor in or on the frame of the straining machine. The tracks are defined by rails which are disposed at two different levels, and the mobile grate is caused to travel relative to the rails in response to actuation of one or more hydraulic or pneumatic motors. The arrangement is such that the rollers are caused to move forwardly and upwardly along switching devices and onto the upper rails during a first stage of a complete cycle in order to advance the grate bars of the mobile grate counter to the direction of liquid flow and forwardly beyond the bars of the stationary grate (in order to lift the intercepted solid constituents off the stationary grate) and thereupon upwardly (in order to move the lifted off constituents toward the upper end of the screen). When the rollers reach the upper ends of the upper rails, they drop onto the respective lower rails with attendant generation of pronounced noise. Moreover, and since the mobile grate is rather heavy, repeated impacts upon the lower rails contribute to extensive wear and shorten the useful life of the machine. The rollers thereupon descend along the lower rails to retract the mobile grate to its starting position as well as to move such grate rearwardly, i.e., the descending mobile grate is moved out of contact with intercepted solid constituents at the exposed side of the stationary grate.
Another drawback of the just discussed machine is that the lower rails and the switching device are located below the liquid level. Therefore, and particularly if the liquid is raw sewage which carries floating as well as submerged solid matter, submerged solid matter accumulates on the lower rails and in and on the switching devices to cause a jamming of the rollers or to compel the hydraulic or pneumatic prime mover or prime movers to exert a large force in order to move the rollers along their prescribed paths. Partial jamming (for example, at one side of the screen) can cause extensive deformation of the screen and/or of the rails and/or other (even more serious) damage to the machine or to certain of its parts.
Swedish Pat. No. 436 416 discloses a modified straining machine wherein the bars of the mobile grate are assembled into a rigid package, and such grate is moved along a path having at least one vertical component in order to ensure that the solid constituents which are lifted off the stationary grate will be moved to a higher level prior to being redeposited on the stationary grate. The path is an endless path, and the upper end of the mobile grate is suspended from eccentric discs. The discs are coupled to each other by chains and are driven by a motor. There is no lateral guidance of the mobile grate; therefore, movements of the suspended mobile grate are unpredictable. In addition, the extent of movability of the mobile grate is relatively small because the dimensions of the eccentric discs cannot be increased at will. Still further, the initial and maintenance cost of the patented apparatus are very high.
In accordance with a further prior proposal, the mobile grate is movable relative to the stationary grate by a hydraulic cylinder and piston unit which is suspended from an overhead trolley. The trolley is moved back and forth, in and counter to the direction of liquid flow, and the cylinder and piston unit moves the suspended grate up and down relative to the stationary grate. The sides of the mobile grate are provided with followers which track substantially elliptical cams in order to prevent undesirable changes in orientation of the mobile grate. The just described straining machine exhibits the drawback that the mechanism for guiding, supporting and moving the mobile grate is very complex, expensive and prone to malfunction.